Two influential human-rights groups say they have freshly documented dozens of civilian deaths in U.S. drone strikes in Pakistan and Yemen, contradicting assertions by the Obama administration that such casualties are rare.

In Yemen, Human Rights Watch investigated six selected airstrikes since 2009 and concluded that at least 57 of the 82 people killed were civilians, including a pregnant woman and three children who perished in a September 2012 attack.

In Pakistan, Amnesty International revisited nine suspected U.S. drone strikes that occurred between May 2012 and July 2013 in the territory of North Waziristan. The group said it found strong evidence that more than 30 civilians were killed in four of the attacks.

The basic circumstances of each of the drone strikes had been previously reported by local and international news outlets. But the human-rights groups said they were able to shed further light on each of the incidents by interviewing survivors, other witnesses and government officials in both countries.

Most drone strikes in Pakistan and Yemen occur in geographically remote areas that are often hostile to outsiders, making independent assessments extremely difficult.

The groups' findings coincide with another report released Friday by a United Nations human-rights investigator, who estimated that 2,200 people have been killed in drone strikes over the past decade in Pakistan alone.

Of those casualties, at least 400 were civilians and another 200 were “probable noncombatants,” according to the U.N. official, Ben Emmerson. He said the statistics were provided to him by Pakistan's Foreign Ministry.

The U.S. government almost never publicly acknowledges its role in individual drone strikes and its legal justification for targeting specific people is shrouded in secrecy.

Partly as a result, estimates of drone-related casualties vary wildly. Sorting out how many people were legitimate targets under the laws of war and how many were bystanders is an even greater challenge.

In their reports, Human Rights Watch and Amnesty International both called on the Obama administration to make its drone-targeting policies more transparent and to publicly investigate reported civilian casualties.

“The full picture will only come to light when U.S. authorities fully disclose the facts, circumstances and legal basis for each of its drone strikes,” Amnesty International concluded in its report, titled, “Will I Be Next? U.S. Drone Strikes in Pakistan.”

Caitlin Hayden, a White House spokeswoman, declined to comment on the reports' findings. But she cited a speech given by President Barack Obama in May, when he announced newly narrowed guidelines for drone attacks. Obama said drones would only be used against people who pose a “continuing, imminent threat” to the United States and only in cases in which the avoidance of civilian casualties would be “a near-certainty.”

“As the President emphasized, the use of lethal force, including from remotely piloted aircraft, commands the highest level of attention and care,” Hayden said in an e-mail.

Drone strikes in Pakistan are carried out by the CIA under a covert program. In Yemen, the CIA and the military's Joint Special Operations Command both conduct drone attacks. Spokespersons for the CIA and Pentagon declined to comment.

Amnesty International highlighted a July 6, 2012, drone attack in the village of Zowi Sidgi, near the city of Miram Shah, in which it said 18 civilians — including a 14-year-old boy — were killed.

In that case, a group of male laborers had gathered in a tent for dinner when a missile blast killed 10 of them. A few minutes later, as rescuers arrived on the scene to treat the wounded, another round of missiles killed another eight people, according to Amnesty.

In Yemen, Human Rights Watch singled out a Sept. 2, 2012, airstrike in the village of Sarar that blew up a minibus, killing 12 passengers, including three children and a pregnant woman. The group said the Yemeni government, which works closely with U.S. counter-terrorism forces, later admitted the attack had been a mistake and compensated families of the victims.

In most of the other drone strikes cited in the reports, the human-rights groups acknowledged that the scenarios were much less clear cut. They acknowledged that many of those who died were suspected members or al-Qaida or the Taliban. In other instances, civilians died alongside armed combatants.

But in virtually all cases, they said, it was impossible to know whether the targets met Obama's threshold for posing an imminent threat to the United States because U.S. officials have kept that information a secret.